Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $61.97   8.452%   $67.2077   $67.21   $67.20   $67.20  
  Purchase #2 $84.36   10.643%   $93.3384   $93.34   $93.35   $93.30  
  Purchase #3 $144.38   11.797%   $161.4125   $161.41   $161.40   $161.40  
  Purchase #4 $56.63   9.146%   $61.8094   $61.81   $61.80   $61.80  
  Purchase #5 $190.58   8.260%   $206.3219   $206.32   $206.30   $206.30  
  Purchase #6 $118.97   7.507%   $127.9011   $127.90   $127.90   $127.90  
  Purchase #7 $75.35   6.702%   $80.4   $80.40   $80.40   $80.40  
  7 purchase totals = $732.24       $798.3910   $798.39   $798.35   $798.30